1. Field of the Invention
The present invention is directed to an improved leak detection system and method for detecting a leak and specifically to a leak detection system for use with a gasoline fluid dispenser and pump.
2. Description of the Prior Art
Submersible gasoline pumps were first introduced in service stations in the mid-1950s. Following this introduction, it was quickly recognized that by putting gasoline underground, under pressure, environmental harms could result if leaks occurred in the system. Almost as a necessity to sell submerged pumps, line leak detectors were developed. The early models were cumbersome devices which shut off the flow of gasoline if leaks were detected. At that time, there was little concern for the environment, and the cost of the gasoline was considerably less than today. Consequently, if a leak was detected and the flow of gasoline cutoff, the leak detection device was usually blamed and quickly removed as a solution to the problem.
In the early 1960s, more acceptable leak detectors were beginning to be introduced. The early leak detectors took up to 15 seconds to make a line pressure test. No gasoline could be dispensed during that time, which proved to be a problem at busy stations and forced the development of quicker acting devices. Additionally, there was a need for leak detectors which only signaled the leaks and did not stop the serving process. Further, there was an urgent need for leak detectors which were unaffected by pressure changes in the absence of a leak.
The advent of self-service operations forced better technology upon the leak detection systems. These newer leak detector systems operated faster than previous models; however, there was and still is a need for a more precise leak detection system. The problem with prior leak detection systems is not necessarily in the system itself but in its use with certain fluids, especially gasoline. Gasoline, under pressure, in a submerged piping system is influenced by a number of factors, among them, mechanical and hydraulic resilience, thermal contraction and, of course, leaks.
Mechanical resilience is typically caused by the movement under stress of gaskets, check valve seats, diaphragms, etc., due to elasticity. Under normal conditions with all parts of the system working properly, mechanical resilience should not be a major problem.
Hydraulic resilience occurs when fuel is agitated and air becomes entrained in the gasoline. Because air is compressible and gasoline is relatively incompressible, this condition can cause a major problem with leak detection systems which rely on a pressure drop in the system to indicate a leak. The decompression of entrained air reduces the rate at which the pressure will reduce, whatever the cause. Additionally, leak detection systems that utilize a predetermined flow rate will be influenced by the compression of entrained air or vapors within the system.
Thermal contraction is perhaps the least understood of the conditions which can cause a loss of pressure in an underground piping system. Thermal contraction occurs when the temperature of the gasoline in the underground storage tanks is higher than the temperature of the pipelines or dispensers. In the fall and winter, the temperature of the gasoline in an underground storage tank can be considerably higher than the temperature of the surrounding piping, and thermal contraction can occur which will reduce the pressure to zero very quickly.
Examples of prior art leak detectors include U.S. Pat. No. 2,979,939 to Shuh, which is directed to a leak detection system in which the leak detector relies upon a flexible diaphragm. The diaphragm moves in the presence of a leak, activating an alarm switch. This device does not completely shut off the flow of gasoline, but just restricts the flow. U.S. Pat. No. 3,183,723 to Deters discloses an apparatus for detecting leaks in a gasoline delivery system by metering a predetermined flow rate into the system. U.S. Pat. No. 3,261,201 to Potash is directed to a leak detector including a pressure switch with normally open contact switches installed in the dispenser. Upon a decrease in fluid pressure, indicating a leak, a diaphragm in the switch causes the contacts to close, thus activating indicator lights.
U.S. Pat. No. 3,541,283 to Milo is directed to a leak detector which is connected to a pump discharge line. The detector includes a sealed bowl enclosing a vertically movable valve. Also within the bowl is a float, which has an electrical switch attached to it. Under normal conditions, the bowl is filled with liquid, indicating the absence of a leak. When a leak occurs, the liquid level drops, dropping the float which activates an electrical switch. U.S. Pat. No. 3,910,102 to McLean is directed to a portable leak detector designed for above-ground use. This detector measures the hydraulic pressure of the system.
U.S. Pat. Nos. 3,935,567 to Reynolds and 4,109,512 to Ladeen, et al. disclose systems for detecting leaks in a gasoline dispensing operation utilizing an indicator which is designed to measure a predetermined drop in pressure. The drop in pressure activates a control circuit enabling an indicator alarm to turn on.
U.S. Pat. No. 4,518,955 to Meyer and U.S. Pat. No. 4,651,559 to Horigome, et al. utilize control circuitry. Meyer is directed to a leak detector controlled by electronic control devices The detector includes a tube in which a movable piston is placed against a spring. There are two sensors located near the inlet position of the tube. Under normal conditions, the piston is near the inlet end, deactivating the sensors. This indicates an absence of fluid. A pressure drop in the system, indicating a leak, causes the piston to move against the spring which activates the sensors Horigome, et al. are directed to a leak detector, which detects leaks by measuring the difference between the upstream and downstream pressure gradient A change in the pressure gradient indicates the occurrence of a leak. The detection system utilizes a plurality of pressure gauges and computer circuitry to compute the pressure gradient of the gas.
While many of the above-noted patents will operate under certain conditions, they are adversely affected by the changes which occur and which are totally unrelated to leaks in the system. There is therefore a need for an accurate leak detector system which is unaffected by changes in mechanical or hydraulic resilience and thermal contraction situations.